1. Field of the Invention
The present invention generally relates to electronic circuits and more specifically to the protection of a switch for controlling a load powered by an A.C. voltage against possible overheating. The present invention more specifically applies to bidirectional switches which are not controllable to be turned off.
2. Discussion of the Related Art
FIG. 1 is a schematic block diagram of an example of a system of powering of a load 1 (M) by an A.C. voltage Vac, for example, the electric supply network mains voltage. Load 1 is controlled by a bidirectional switch 10 (for example, a triac) with which it is series-connected between two terminals 2 and 3 of application of voltage Vac. Switch 10 is not controllable to be turned off in that, while it turns off by removal of the current that it conducts, it cannot be turned off by an action on its control terminal. Switch 10 is associated with a control circuit 20 (CTRL PART) in an integrated circuit. Circuit 20 may be controlled by a digital unit 30 (MCU) which then receives one or several signals from circuit 20 (especially a signal indicating the state of switch 10) and provides it with one or several control signals. Control circuit 20 and unit 30 are powered by a D.C. voltage Vcc applied, in this example, between terminal 2 and a ground terminal 4.
Switch 10 and its control circuit 20 are placed in a same package 5 having external connection terminals connected to the rest of the assembly. In this example, two terminals 51 and 52, connected inside of package 5 to the power electrodes of triac 10, are respectively connected to terminal 2 and to a first terminal of load 1. Two terminals 53 and 54 represent the supply terminals of control circuit 20 and are thus respectively connected to terminals 2 and 4. At least two terminals 55 and 56 are connected to control unit 30.
FIG. 2 is a simplified view of a packaged circuit 5 comprising switch 10 and its control circuit 20. In this example, triac 10 is a monolithic component placed on a support 57 next to integrated circuit 20. Gate G of the triac is connected by a conductor 58 to the corresponding terminal 21 (FIG. 1) of control circuit 20. The assembly is generally encapsulated in a resin 59 to form circuit 5.
In applications aimed at by the present invention, control circuit 20 comprises a temperature detection circuit intended to detect a heating up of switch 10, for example, to detect an overcharge. Such a temperature detection circuit compares information representative of the switch temperature (for example, a junction temperature) with a threshold to detect a possible exceeding thereof. The function of this temperature detection is to inhibit the turning on (at the next halfwave) of switch 10 to avoid for it to be damaged.
The detector is either placed next to switch 10, as illustrated in FIG. 2, by being integrated to circuit 20, or directly placed on the component forming the switch. In this case, the detection result is provided to circuit 20 and/or to circuit 30.
FIG. 3 schematically shows an example of a conventional thermal detector. This circuit generally belongs to circuit 20 (FIG. 1). A comparator 41 (COMP), having a first input receiving the result of a measurement of a junction temperature Tj performed by a detector 42 and having a second input receiving a temperature threshold, provides a two-state signal OVT, indicative of whether threshold TH has been exceeded by the measured current temperature Tj.
FIG. 4 shows an example of an electric diagram of thermal detector 42 shown in FIG. 3. Comparator 41 is a differential amplifier having an input (for example, inverting) receiving a temperature-stable voltage Vth of band-gap type, and having its other input (for example, non-inverting) receiving a voltage Vptat proportional to the integrated circuit temperature. Voltage Vptat is sampled from the junction point of a PNP-type transistor Q5 and of a resistor Rb. Grounded resistor Rb is used as a current-to-voltage converter of a current Iptat proportional to the temperature corresponding to the collector current of transistor Q5. The emitter of transistor Q5 is connected to terminal 53 of application of supply voltage Vcc by means of a resistor Re3. Transistor Q5 copies the current of a so-called ΔVbe/R current source. This source is formed of two parallel branches between terminal 53 and the ground. A first branch comprises, in series, an emitter biasing resistor Re1, a PNP-type bipolar transistor Q3, and an NPN-type bipolar transistor Q1. A second branch comprises, in series, an emitter biasing resistor Re2, a PNP-type bipolar transistor Q4, an NPN-type bipolar transistor Q2, and an emitter resistor Ra. Transistors Q1 and Q4 are diode-assembled. Transistor Q4 is mirror-assembled on transistor Q3 and transistor Q1 is mirror-assembled on transistor Q2. Assuming that transistor Q5 has the same surface area (1) as transistor Q4, voltage Vptat is a function of temperature T, provided by the following relation:
      Vptat    =                            Re          ⁢                                          ⁢          2                          Re          ⁢                                          ⁢          3                    ·              Rb        Ra            ·                        k          ·          T                Q            ·              Ln        ⁡                  (                                    a              ⁢                                                          ⁢                              2                ·                a                            ⁢                                                          ⁢              3                                      a              ⁢                                                          ⁢                              1                ·                a                            ⁢                                                          ⁢              4                                )                      ,
where a1, a2, a3, and a4 represent the respective surface areas (1, 8, 1, and 1 in this example) of transistors Q1 to Q4, k designates Boltzmann's constant, and Q designate the electron charge.
The only variable in the above relation is, for a given circuit, temperature T.
To start the circuit, a transient current needs to be applied to the ΔVbe/R source. For this purpose, a start-up circuit 45, formed of a resistor R′ in series with a PNP-type bipolar transistor Q6, connects terminal 53 to the junction point (common collector) of transistors Q3 and Q1. The base of transistor Q6 receives a turn-on control pulse to start a measurement. This pulse is provided, for example, by unit 30.
The more the temperature increases, the more voltage Vptat increases. Since voltage Vth is stable in temperature, it sets a triggering threshold of the comparator. This threshold is adjusted by the ratio between resistors Rb and Ra. Comparator 41 is formed, for example, by means of an operational amplifier.
A disadvantage of the detector of FIG. 4 is that, on initial powering-on of load 1, switch 10 is likely to transiently exceed the triggering temperature. Now, a temperature peak of short duration generally poses no problem and should not be considered as an overheating. However, the output of comparator 41 risks inhibiting the turning-on of switch 10.